Documents EP-A-0544962 and WO 03/044499 disclose an infrared imager associated with optical measurement and reference filters that are placed in succession on the optical axis of the imager and that have passbands containing an absorption line of a looked-for gas (for the measurement filters) or that are complementary to said absorption line (for the reference filters). The background of the zone under observation is used as a source of infrared and the presence of a looked-for gas is revealed by differential processing of the infrared images taken through the filters, with the processing making it possible to calculate the concentration of the detected gas.
In practical manner, a set of measurement and reference filters is carried by a motor-driven rotary disk so as to bring the filters in succession onto the optical axis of the imager. The images of the observed zone in the various spectral bands corresponding to the passbands of the filters are acquired sequentially.
That type of device thus enables a given gas to be looked for and analyzed in a zone of space towards which the imager is pointed. Such a device requires prior calibration using a background that emits standard radiant flux when no gas is present. Nevertheless, that type of calibration is found to be relatively inaccurate because of the difficulty of defining a standard background, which background will always be different from the real background, thereby greatly limiting the accuracy of gas concentration measurements. Thus, in practice, that type of device serves essentially for identifying the presence of a given gas, but does not make it possible to indicate accurately its concentration in the zone of space under observation. Furthermore, identifying a given gas requires a measurement filter with a corresponding absorption line to be available, which implies that analyzing a mixture of gases, having a plurality of different absorption lines, is difficult to achieve. The analysis of absorption lines of different chemicals in the zone under observation is limited by the number of filters used. Different chemicals may present absorption lines that are similar.